Hi,
With [1] now merged, the extension phpunit tests that are run by jenkins
will also validate loaded extension.json and skin.json files against the
formal JSON schema (docs/extension.schema.json in core).
To validate locally, you can either run the structure phpunit tests or
use the validateRegistrationFile.php maintenance script.
[1]
https://gerrit.wikimedia.org/r/#/c/273751/
-- Legoktm